The goal of this project is to develop a general, modular fiber optic sensor system capable of quantifying biologically relevant levels of specific proteins in vitro. The sensor system is designed for rapid bedside analysis, for primary diagnoses in rural or technologically poor environments, and timely diagnoses by first responders in the field. The sensor system will be faster than existing commercially available fieldable sensors while maintaining the sensitivity and selectivity of common laboratory tests for the target proteins. By constructing the sensors on optical fibers, the sensor system can employ inexpensive, interchangeable sensing elements for specific tests with simple automated procedures for preparing the sensing elements for operation. The sensors use surface plasmon resonance (SPR) on optical fibers to detect the markers through immunoassays, and are encapsulated in a thin film polymer housing that prevents cellular fouling of the sensing region. Phage display will be employed to collect improved protein specific receptors with desired binding sensitivities for target applications. Because the assay is conducted on the surface of an optical fiber, the probes can be used as immersion probes in vitro. Four target example applications around which we will base development and optimization of the sensor system have been chosen: proteins for detecting and determining the severity of myocardial infarctions, proteins for assessing ability of wounds to heal, proteins associated with ischemic stroke; proteins for detecting spinal motor atrophy. Sensor optimization will address specific biological matrices where these proteins are found. These example applications are selected to demonstrate the flexibility and relevancy of the sensing system, but should not be construed as the limiting cases of its use. In fact, the antibody-based immunoassay approach has applications far beyond those considered here.